The phylum of the chords, from the Latin Chordata, is composed of animals that have a notochord – a kind of flexible rod that runs longitudinally along the animal’s back at some point in life – indicating the existence of a common ancestor. Vertebrates, a group included in the phylum of the chordates, have the spine in place of the notochord. The animals of this phylum also have a nervous tube located dorsally, above the notochord. In addition, they also have a post-anal tail – which is important for aquatic beings and, in humans, is called the tailbone, the vestige of that tail – and gill slits in the pharynx, which are embryonic structures, and can remain in adulthood or to vanish. In aquatic species, they persist throughout life and have a respiratory function, whereas in terrestrial ones, they disappear giving rise to the trachea, a structure used for breathing in these animals.
All characteristics, unlike invertebrates, were found in animals in their embryonic phase, so during their development some of the characteristics may disappear.
Subphiles in the phylum Chordata
The phylum has three subphyla:
Urochordata or Tunicata
In this sub-phylum of the Urochordata, also recognized as tunicates – due to the wrapping of their body which is composed of a thick tunic – we have as the best known the ascidians, which are marine chordates that live both isolated and in colonies. As in the amphioxus – topic to follow – these animals have well developed gill slits. Ascidians are free and native during the larval phase, but when adults are fixed. Its feeding is done through the water that enters the animal through the inhaling siphon.
Many of the tunicates are sessile, as are the ascidians, living fixed and filtering particles due to the current of water they produce. However, there are others, such as salps, which are quite small and are grouped into gelatinous colonies, but which move through the currents.
Cephalochordata – amphioxus
The cephalocordates can be easily represented by the animals that we know by the name of amphioxus – name derived from the animals having the body tapered at two ends, because amphi = two -. With approximately thirty species, amphioxus live in the sea. With a small size – up to 8 cm long – these animals have a body similar to that of a fish, but live semi-buried in the sand keeping only the front part out, usually in places with clean and calm waters. In cephalocordates, fertilization is external and the circulatory system is formed by vessels – the contractile ones being responsible for the propulsion of the blood, as they do not have a heart.
Vertebrata or craniata
Vertebrate animals comprise agates, fish, amphibians, reptiles, birds and mammals, and are characterized by the segmented spine, in addition to the skull, which protects the brain. They also have bilateral symmetry – existence of a symmetrical muscular system – and central nervous system. Lampreys, the oldest descendants of vertebrates, do not have a mandible, but an opening surrounded by teeth that they use to attach themselves to the skin of other animals. Bone fish are found in both fresh and salt water. Reptiles and mammals are descended from the same ancestor, but reptiles have diversified a lot, originating flying, swimming and terrestrial reptiles, whereas mammals were a group of small animals similar to today’s rodents